Computer Networking: A Top Down Approach Featuring the Internet - Chapter 3 Transport Layer ppt

Chapter 3
Transport Layer

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All material copyright 1996-2006
J.F Kurose and K.W. Ross, All Rights Reserved

Computer Networking:
A Top Down Approach
Featuring the Internet,
3rd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2004.

Transport Layer 3-1


Chapter 3: Transport Layer
Our goals:

Ì understand
principles
behind transport
layer services:
r

r
r
r

multiplexing/demultipl
exing
reliable data transfer
flow control
congestion control

Ì learn about

transport layer
protocols in the
Internet:
r

r

r

UDP: connectionless
transport
TCP: connection-oriented

transport
TCP congestion control

Transport Layer 3-2


Chapter 3 outline
Ì 3.1 Transport-

layer services
Ì 3.2 Multiplexing
and
demultiplexing
Ì 3.3
Connectionless
transport: UDP
Ì 3.4 Principles of
reliable data
transfer

Ì 3.5 Connection-

oriented transport:
TCP
r
r
r
r

segment structure

reliable data transfer
flow control
connection management

Ì 3.6 Principles of

congestion control
Ì 3.7 TCP congestion
control
Transport Layer 3-3


Transport services and protocols
Ì provide logical

network
data link
physical

network
data link
physical

ca
gi
lo

le

nd

-e
nd

network
data link
physical

a
tr

network
data link
physical

network
data link
physical

rt
po
ns

communication between
app processes running on
different hosts
Ì transport protocols run
in end systems
r send side: breaks app
messages into segments,
passes to network layer

r rcv side: reassembles
segments into messages,
passes to app layer
Ì more than one transport
protocol available to
apps
r Internet: TCP and UDP

application
transport
network
data link
physical

application
transport
network
data link
physical

Transport Layer 3-4


Transport vs. network layer
Ì network layer:

logical
communication
between hosts
Ì transport layer:

logical
communication
between processes
r

relies on, enhances,
network layer services

Household analogy:
12 kids sending
letters to 12 kids
Ì processes = kids
Ì app messages =
letters in envelopes
Ì hosts = houses
Ì transport protocol =
Ann and Bill
Ì network-layer
protocol = postal
service

Transport Layer 3-5


Internet transport-layer protocols
Ì reliable, in-order

delivery (TCP)
r

no-frills extension of
“best-effort” IP

Ì services not

network
data link
physical

network
data link
physical

rt
po
ns

r

network
data link
physical

a
tr

unordered delivery:
UDP

nd

-e
nd

Ì unreliable,

network
data link
physical

le

r

congestion control
flow control
connection setup

network
data link
physical

ca
gi
lo

r

application
transport
network

data link
physical

application
transport
network
data link
physical

available:
r
r

delay guarantees
bandwidth guarantees

Transport Layer 3-6


Chapter 3 outline
Ì 3.1 Transport-

layer services
Ì 3.2 Multiplexing
and
demultiplexing
Ì 3.3
Connectionless
transport: UDP
Ì 3.4 Principles of

reliable data
transfer

Ì 3.5 Connection-

oriented transport:
TCP
r
r
r
r

segment structure
reliable data transfer
flow control
connection management

Ì 3.6 Principles of

congestion control
Ì 3.7 TCP congestion
control
Transport Layer 3-7


Multiplexing/demultiplexing
Multiplexing at send host:
gathering data from multiple
sockets, enveloping data with
header (later used for

demultiplexing)

Demultiplexing at rcv host:
delivering received segments
to correct socket
= socket
application
transport
network
link

= process
P3

P1
P1

application
transport
network

P2

P4

application
transport
network
link

link

physical

host 1

physical

host 2

physical

host 3

Transport Layer 3-8


How demultiplexing works
Ì host receives IP datagrams

each datagram has source IP
address, destination IP address
r each datagram carries 1
transport-layer segment
r each segment has source,
destination port number
Ì host uses IP addresses & port
numbers to direct segment to
appropriate socket
r

32 bits
source port #

dest port #

other header fields

application
data
(message)
TCP/UDP segment format

Transport Layer 3-9


Connectionless demultiplexing
Ì Create sockets with

port numbers:
DatagramSocket mySocket1 = new
DatagramSocket(12534);
DatagramSocket mySocket2 = new
DatagramSocket(12535);

Ì UDP socket identified

by
(dest

two-tuple:

IP address, dest port
number)

Ì When host receives

UDP segment:
r

r

checks destination port
number in segment
directs UDP segment to
socket with that port
number

Ì IP datagrams with

different source IP
addresses and/or
source port numbers
directed to same
socket

Transport Layer 3-


Connectionless demux (cont)

DatagramSocket serverSocket = new DatagramSocket(6428);
P2

SP: 6428
DP: 9157

client
IP: A

P1
P1

P3

SP: 9157
DP: 6428

SP: 6428
DP: 5775

server
IP: C

SP: 5775
DP: 6428

Client
IP:B

SP provides “return address”

Transport Layer 3-


Connection-oriented demux
Ì TCP socket

identified by 4tuple:
r
r
r
r

source IP address
source port number
dest IP address
dest port number

Ì recv host uses all

four values to
direct segment to
appropriate socket

Ì Server host may

support many
simultaneous TCP
sockets:
r

each socket identified by
its own 4-tuple

Ì Web servers have

different sockets
for each connecting
client
r

non-persistent HTTP will
have different socket for
each request

Transport Layer 3-


Connection-oriented demux
(cont)
P1

P4

P5

P2

P6

P1
P3

SP: 5775
DP: 80
S-IP: B
D-IP:C

client
IP: A

SP: 9157
DP: 80
S-IP: A
D-IP:C

server
IP: C

SP: 9157
DP: 80
S-IP: B
D-IP:C

Client
IP:B

Transport Layer 3-

Connection-oriented demux:
Threaded Web Server
P1

P2

P4

P1
P3

SP: 5775
DP: 80
S-IP: B
D-IP:C

client
IP: A

SP: 9157
DP: 80
S-IP: A
D-IP:C

server
IP: C

SP: 9157
DP: 80
S-IP: B

D-IP:C

Client
IP:B

Transport Layer 3-


Chapter 3 outline
Ì 3.1 Transport-

layer services
Ì 3.2 Multiplexing
and
demultiplexing
Ì 3.3
Connectionless
transport: UDP
Ì 3.4 Principles of
reliable data
transfer

Ì 3.5 Connection-

oriented transport:
TCP
r
r
r
r

segment structure
reliable data transfer
flow control
connection management

Ì 3.6 Principles of

congestion control
Ì 3.7 TCP congestion
control
Transport Layer 3-


UDP: User Datagram Protocol

[RFC 768]

Ì “no frills,” “bare

bones” Internet
transport protocol
Ì “best effort” service,
UDP segments may be:
r lost
r delivered out of order to
app
Ì connectionless:
r no handshaking between
UDP sender, receiver

r each UDP segment
handled independently of
others

Why is there a UDP?
Ì no connection

establishment (which
can add delay)
Ì simple: no connection
state at sender,
receiver
Ì small segment header
Ì no congestion control:
UDP can blast away as
fast as desired

Transport Layer 3-


UDP: more
Ì often used for

streaming multimedia
apps
Length, in
r loss tolerant
bytes of UDP
r rate sensitive
segment,

32 bits
source port #

dest port #

length

checksum

including

Ì other UDP uses
header
r DNS
r SNMP
Ì reliable transfer over
UDP: add reliability
at application layer
r application-specific
error recovery!

Application
data
(message)
UDP segment format

Transport Layer 3-

UDP checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment
Sender:

Receiver:

Ì treat segment

Ì compute checksum of

contents as sequence
of 16-bit integers
Ì checksum: addition
(1’s complement sum)
of segment contents
Ì sender puts checksum
value into UDP
checksum field

received segment
Ì check if computed
checksum equals checksum
field value:
r NO - error detected
r YES - no error detected. But
maybe errors nonetheless?
More later ….

Transport Layer 3-

Internet Checksum Example
Ì Note
r

When adding numbers, a carryout from the most
significant bit needs to be added to the result

Ì Example: add two 16-bit integers

1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0
1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0
checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1

Transport Layer 3-


Chapter 3 outline
Ì 3.1 Transport-

layer services
Ì 3.2 Multiplexing
and
demultiplexing
Ì 3.3
Connectionless
transport: UDP

Ì 3.4 Principles of
reliable data
transfer

Ì 3.5 Connection-

oriented transport:
TCP
r
r
r
r

segment structure
reliable data transfer
flow control
connection management

Ì 3.6 Principles of

congestion control
Ì 3.7 TCP congestion
control
Transport Layer 3-


Principles of Reliable data transfer
Ì important in app., transport, link layers
Ì top-10 list of important networking topics!

Ì characteristics of unreliable channel will determine complexity of reliable

data transfer protocol (rdt)

Transport Layer 3-


Principles of Reliable data transfer
Ì important in app., transport, link layers
Ì top-10 list of important networking topics!

Ì characteristics of unreliable channel will determine complexity of reliable

data transfer protocol (rdt)

Transport Layer 3-


Principles of Reliable data transfer
Ì important in app., transport, link layers
Ì top-10 list of important networking topics!

Ì characteristics of unreliable channel will determine complexity of reliable

data transfer protocol (rdt)

Transport Layer 3-


Reliable data transfer: getting started

rdt_send(): called from above,
(e.g., by app.). Passed data to
deliver to receiver upper layer

send
side

udt_send(): called by rdt,
to transfer packet over
unreliable channel to receiver

deliver_data(): called by
rdt to deliver data to upper

receive
side

rdt_rcv(): called when packet
arrives on rcv-side of channel

Transport Layer 3-


Reliable data transfer: getting started
We’ll:
Ì incrementally develop sender, receiver
sides of reliable data transfer
protocol (rdt)
Ì consider only unidirectional data
transfer

r

but control info will flow on both directions!

Ì use finite state machines (FSM) transition
event causing state to
specify sender, receivertaken on state transition
actions
state: when in this
“state” next state
uniquely determined
by next event

state
1

event
actions

state
2

Transport Layer 3-